This invention relates in general to orthodontic and dental devices and in particular to a new and useful method and apparatus for treating by thermal polymerization plastic elements which are used for dental purposes.
The invention relates particularly to a process and a device for thermal polymerization of plastics for dental purposes in an enclosed space of a pressure receptacle in which a heated medium enclosing the material to be polymerized is placed under excess pressure.
The pressure polymerization equipment currently used in the dental laboratory to make dental prosthetics are only suitable for polymerization processes that take place in a liquid medium such as water or glycerin, heated to a polymerization temperature of 120.degree. C., for example, and simultaneously put under pressure of 6 bar, for example. In the water polymerization process, for each new polymerization procedure the pressure receptacle of the polymerization equipment is usually filled with fresh, cold water in which the material to be polymerized must be completely immersed. With the polymerizates used in such cases, the major factors in obtaining a good result from the polymerization are to maintain the polymerization temperature at exactly the right level and further to make sure that the temperature is evenly distributed throughout the cavity of the pressure receptacle. These requirements can be met easily and thoroughly with water as the medium in which polymerization takes place, because water is a good conductor of heat. On the other hand, however, the use of water as the polymerization medium entails the disadvantage that a relatively large amount of heat energy is required to heat it up to a specific polymerization temperature. Furthermore, it is somewhat tedious to handle in that for the each new polymerization procedure the pressure receptacle has to be refilled with water and then drained again afterwards. Moreover, the use of water as the polymerization medium presupposes a pressure receptacle that can only be opened from the top.
To accomplish slow heating and cooling of the plastic and hence avoid stresses in the molded bodies that have walls of varying thicknesses, a prior art process (German Patent Disclosure No. 2,021,194) of the general type discussed above provides that the molded bodies are brought into contact with the surrounding contact fluid while the fluid is still at a temperature below the polymerization temperature. The molded bodies are then heated together with the contact fluid up to a polymerization temperature and finally cooled slowly. For this purpose, an additional receptacle is provided that is equipped with a heat condenser that is placed in an influx channel placed into the interior space of the receptacle or an influx channel is provided in the interior space of the receptacle.
Apart from the fact that the contact medium used is still a liquid that has to be changed often, an additional receptacle is required to hold the material to be polymerized, which makes the device more expensive and handling is more involved.
In another polymerization process, a predetermined amount of water is put into a heatable pressure receptacle with the material to be polymerized and heated to a vapor after the pressure receptacle is closed. What results is a mixture of hot air and water vapor. This process has not been found to be particularly good in practice, because uneven hardening with uneven degrees of hardness and uneven coloring have been noted. Furthermore, with this process, to an even greater degree than with the polymerization in a water bath, problems at the interface between metal and plastic due to the formation of gaps have been noted. Obviously, with this process, the transfer of heat from the heat source to the material to be polymerized is inadequate.
Recently a device has appeared on the market in the U.S. (Justi Pakto, H. D. Justi Company, Oxnad, CA) consisting of a sort of pressure cooker with a lid and equipped with a gas supply line, manometer, thermometer and a pressure satety valve. The pressure range that can be obtained is between 1.4 and 2.7 bar. The temperature range is between 120.degree. and 204.degree. C. and is achieved by means of a built-in 5-step electric hot plate. Because of the low pressure obtainable, only a single plastic material can be used in this equipment, namely 100% polyglycoldimethacrylate. That material, however, is not comparable in quality with the filled or unfilled plastics more commonly used today for dental work that are improved in wear resistance, impact strength, hardness, color and so forth and come very close to the properties of natural teeth. They do require, however, very process polymerization conditions to be maintained.